The present invention relates to an active matrix type display device, for example, a display device using light emission devices that emit light by themselves, such as organic LEDs.
Demands for personal computers, portable information terminals, information communication equipment or composite products thereof have increased with the advent of a high information society. For use in such products, display devices of reduced thickness and weight and having a high-speed response are in demand.
As a display device which is suitable to meet such demands, displays that are formed by light emission devices that are capable of saving electric power have been proposed. Generally, an active matrix type display device is formed by arranging rectangular pixel regions in a matrix form on a substrate. For example, an organic LED, as one example of light emission devices is formed by putting an organic LED device, comprising a hole transportation layer, a charge injection layer and an organic light emission layer, between a transparent pixel electrode (anode) and a metal pixel electrode (cathode) in such a manner that both surfaces of the organic LED device are in contact with the transparent pixel electrode and the metal pixel electrode, respectively. Existent active matrix type organic LED display devices are formed by disposing a transparent pixel electrode of the organic LED device on the side of the transparent substrate, such as a substrate made of glass and disposing the metal pixel electrode on the side opposite to the substrate.
In the driving circuit for the organic LED device in each pixel, a first thin layer transistor (TFT1) is disposed at a position near the intersection between each of the scanning lines and the signal lines arranged in the form of a lattice, the TFT1 is driven by scanning signals and pixel signals to store data in a capacitor (holding capacitor), a second thin layer transistor (TFT2) is driven in accordance with the voltage of the capacitor and the current flowing to the organic light emitting layer by way of the transparent pixel electrode, which serves as the anode connected with the TFT2, is controlled to emit light. Then, light emitted from the organic light-emitting layer passes out through the transparent pixel electrode on the substrate side.
In the case of a bottom emission type display, in which emission light is taken out from the substrate, since the area of the driving circuit, comprising transistors TFT1, TFT2, the capacitor and lines arranged for each of the pixels, hinders light transmission, an improvement in the so-called aperture ratio is limited.
In view of the above, to improve the aperture ratio without effect on the driving circuits, such as the transistor TFT, a so-called top emission type display device, that takes out light from the side opposite to the substrate, has been proposed (Document: SID2001 Digest-24-4L). In such a top emission type of device, an improvement in the aperture ratio can be expected as compared with the bottom emission type of device. In this top emission type of device, light is taken out by using a transparent electrode layer on the upper side of the organic LED device, but the document referred to above discloses no actual structure of the organic LED device and the driving circuit.
Further, the dielectric constant of the transparent pixel electrode layer is generally greater by about one digit or more as compared with that of the metal pixel electrode layer. Accordingly, the current consumption increases as the size of the display panel becomes larger, which poses a problem in that the power loss of the current supply line to the organic LED device increases.
Further, the organic LED device has the characteristic that it degrades rapidly due to heat or humidity. Accordingly, the method of forming a transparent pixel electrode layer in the upper portion, or a method of patterning the transparent pixel electrode layer, becomes significant. In particular, to realize multi-color display, organic LED devices capable of displaying plural colors are necessary. However, since the light emission characteristics of devices are different from one color to another color at present, it is preferred to separate, for each color, the lines for current supply in order to effectively control the display colors. However, this involves a problem in that it is difficult to form the transparent pixel electrode layer into an optional shape.
A first object of the present invention is to provide a specific pixel structure in a top emission type display device using light emission devices.
A second object of the present invention is to provide a current supply structure to a transparent pixel electrode that is capable of coping with an increase in the scale in the top emission type display device using light emission devices.
A third object of the present invention is to provide a pixel structure that is suitable to coloration of a panel in the top emission type display device using organic LED devices.